The study investigates the surface charge properties of biomolecular condensates and coacervates, which are crucial for understanding their role in cellular organization. Traditional methods like laser Doppler electrophoresis, which assume size-independent electrophoretic mobility, are shown to be inaccurate for liquid-like condensates. The authors introduce a microelectrophoresis platform that tracks individual droplets in an electric field, allowing for the measurement of electrophoretic mobility and zeta potential. This method reveals that coacervate droplets have size-dependent electrophoretic mobility, which is influenced by their radius, Debye length, and internal viscosity. The zeta potential of these droplets is calculated using a modified equation derived from Ohshima et al.'s theory. The study also demonstrates that the surface charge of coacervates can be altered by the addition of ATP, which inverts the surface charge and displaces α-synuclein, a protein that accumulates at the interface. These findings highlight the importance of surface charge in the behavior and function of biomolecular condensates and coacervates, and the microelectrophoresis platform provides a promising tool for further investigation.The study investigates the surface charge properties of biomolecular condensates and coacervates, which are crucial for understanding their role in cellular organization. Traditional methods like laser Doppler electrophoresis, which assume size-independent electrophoretic mobility, are shown to be inaccurate for liquid-like condensates. The authors introduce a microelectrophoresis platform that tracks individual droplets in an electric field, allowing for the measurement of electrophoretic mobility and zeta potential. This method reveals that coacervate droplets have size-dependent electrophoretic mobility, which is influenced by their radius, Debye length, and internal viscosity. The zeta potential of these droplets is calculated using a modified equation derived from Ohshima et al.'s theory. The study also demonstrates that the surface charge of coacervates can be altered by the addition of ATP, which inverts the surface charge and displaces α-synuclein, a protein that accumulates at the interface. These findings highlight the importance of surface charge in the behavior and function of biomolecular condensates and coacervates, and the microelectrophoresis platform provides a promising tool for further investigation.